1. Field of the Invention
The present invention relates to pre-amplifier systems, and more particularly to an improved hydrophone pre-amplifier with self-calibration using differential charge amplifiers.
2. Description of the Prior Art
To develop definitive models of the deep ocean floor and subbottoms, which models are used to predict the performance of marine systems which acoustically interact with the bottom, a deep-towed measurement system using a low-frequency sound source and multi-channel array system as shown in FIG. 1 is required. The low-frequency sound source, together with a digital telemetry system capable of driving a long coaxial tow cable and compatible with downlink power transmission, is mounted on a deep-towed vehicle, or fish. Behind the fish a hydrophone array is streamed. The hydrophone array, as shown in FIG. 2, has a plurality of alternating active sections and interconnect sections. The active sections each contain a plurality of hydrophones. The interconnect sections contain interconnect wiring, a pre-amplifier for each hydrophone in the downstream active section, a depth sensor and a heading sensor. The data acquired is transmitted by the telemetry system on the fish via the tow cable to a towing vessel for data processing.
A charge amplifier design, a simplified circuit of which is shown in FIG. 3, is used for the pre-amplifiers so that the hydrophone signal is not attenuated by the cable capacitance between the hydrophone and the pre-amplifier. Also, the hydrophone/pre-amplifier signal gain is equal to the ratio of the charge amplifier feedback capacitor, C.sub.f, and the hydrophone capacitance, C.sub.H. One disadvantage, however, is that a change in hydrophone capacitance due to depth causes a change in the overall system gain by as much as 10 dB from near surface to 6000 m.